Points pertaining to the management of a new dam, also apply to dams which have been built for some time. In the management of an established dam, the improvements necessary have to be done when it is at its lowest level and at a possible depth of water. Special attention should be paid to weeds in a dam, especially to excessive weeds, as a reasonable amount of water weeds is essential for balanced life in the water.
Aquatic weeds may be divided broadly into three groups:
- Hard vegetation:Plants with leaves and stems above the water, and rooted in shallower parts of the dam. Bulrushes, reeds and water sedge belong to this group. These are a hindrance to fishing and take away valuable nutrients from the dam.
- Semi-emergent weeds: Stems reaching the water surface, leaves and flowers usuallyfloating on the surface. Water lilies, Elodea, and Pondweed (potamogeton) belong to this group. These grow in slightly deeper water and may cover large areas if not brought under control.
- Submerged weeds: Entirely under-water growing plants, of which Nitella is the most common and usually outgrows all other plants in this group. (Myriophyllum Lagarosiphan a.c.).
Certain amounts of aquatic plants are important in dams, because they provide both shelter and food for the microscopic animals upon which the young fish feed and they provide cover for the fry. Submerged plants produce oxygen which is essential for aquatic animal life and lastly the decaying plants increase the food for plankton.
Uncontrolled growth of the weeds, on the other hand, may make dam management very difficult or completely impossible. The weeds can choke up the entire dam surface, preventing fishing of any kind, and also use some of the nutrients in the water. Furthermore, they promote bilharzia carrying snails.
The control of this excessive vegetation is not an easy task, especially when they have been left to expand for many years without any preventative steps being taken.
Three methods available to eradicate aquatic vegetation are:
Chemical control is done by spraying the plants with some herbicide such as 2 – 4D in tributyl solvent and kerosene carrier, diquat and some others which have negligible effect on fish and human beings. In some cases by using weed killers with very good primary results the removed dominant water weeds have by nature been later replaced with more prolific and resistant species.
Biological control includes the stocking of dams with weed eating kinds of fish, keeping water birds on the dam and fertilizing the water to the safe limit which will increase the growth of phytoplankton, resulting in the decrease of the transparency of water and thus the growth of submerged weeds.
Mechanical methods include cutting the plants, pulling out by hand and ploughing the soil in dried-up areas. For cutting, a scythe attached to a long handle, swivel mounted knives on a boat or weed cutting chains with sharp blades can be used. A very effective cutting device can be made using old mower blades joined together with wire and facing alternate directions.
To keep dams clear of excessive weeds, control measures should start in newly constructed dams. Where the growth of weeds has by negligence become too dense, steps should be taken to bring it under control again. The best results may be achieved by using combined methods, as mentioned above. The use of weed killers should follow with the stocking of the dam with weed eating fish. Cutting out a certain amount of weeds may help the fish to bring the plants under control and ducks on a dam not only eat the weeds but also help to fertilize the water.
Farm dams are not built primarily for fish culture but they can be an important source of protein. Fish production in dams depends largely on natural factors, such as site of dam, character of catchment area, size and depth of dam and of the in and out flow of water. The best species of fish for farm dams are tilapia mossambica and tilapia melanopleura, stocked at the rate of 1 – 2kg fingerlings or mixed sizes per ha.
Fishing is most effectively done by seine and gill netting, but traps and hook gears have their place in harvesting the dams.
Water weeds, although necessary for balanced life in water, should be kept under control in dams so that they do not prevent successful fishing whether it is done for profit or pleasure.